Camera-based method for calibrating color displays

ABSTRACT

Provided herein are teachings directed to overcoming the problem of erroneous color reproduction on a color output device such as a color display. The teachings herein provide a method for correcting color image data input to a display device by displaying a target of color patches of known input values on the display device, and capturing an image of the target with a digital camera. This is followed by extracting camera signals from the image which corresponds to the color patches, and deriving a tone response calibration for the projector from the camera signals and the input values.

CROSS-REFERENCE TO RELATED APPLICATIONS

Cross reference is made to the following application file concurrentlyherewith: Ser. No. 11/012,993 entitled “A Camera-Based System ForCalibrating Color Displays” to inventors, Raja Bala, Karen M. Braun, andRobert J. Rolleston, the disclosure of which is totally incorporated byreference herein. The appropriate components and processes of the aboveco-pending application may be selected for the invention of the presentapplication in embodiments thereof.

BACKGROUND AND SUMMARY

The teachings presented herein relate generally to calibration of outputdevices. The teachings presented herein relate more specifically tocalibration of color displays.

An ever increasing number of presentations today are givenelectronically using projection display technology. However, in suchpresentations, color images often do not reproduce correctly due to lackof projector calibration. In cases where the color imagery is intendedto convey an important message, this problem can severely diminish thevalue of a presentation. Examples include technical, educational, andmarketing presentations attempting to demonstrate color and imagequality effects. Readability of text and other details are oftencompromised as well, and often the presenter is compelled to apologize,insisting, “It looked good on my computer screen.”

A standard approach for determining the projector's tone response is tomake device-independent measurements of R, G, B ramps with aspectroradiometer, and then derive a tone response function that relatesdigital input value to luminance by fitting or interpolating themeasured data. This type of approach can be expected to produce a highlyaccurate correction. However, making spectroradiometric measurements isa very expensive, time-consuming and tedious process. Indeed this is thereason why projection display calibration is typically avoided, andusers simply live with and otherwise tolerate the erroneous result.

What is needed is a straight forward easy to perform calibration forprojection displays which does not require expensive test equipment orspecial involved operator skills or training to accomplish.

Disclosed in embodiments herein is a method of correcting colors inputto an output device comprising, rendering a target of color patches ofknown input values on the output device, and capturing an image of thetarget with a digital camera. This is followed by extracting camerasignals from the captured image corresponding to the color patches, andderiving a tone response calibration for the output device from thecamera signals and the known input values.

Further disclosed in embodiments herein is a method of color correctionfor a display device comprising, displaying a target of patches of knowninput values on the display device; and capturing an image of the targetof patches with a digital camera. This is followed by calibrating thedigital camera as based upon the captured image of the target patches;extracting calibrated camera signals from the captured image of thetarget of patches, the extracted calibrated camera signals correspondingto the patches in the target of patches; and deriving a tone responsecalibration for the display device from the extracted calibrated camerasignals and the known input values.

Further disclosed in embodiments herein is a method of color correctionfor a display device comprising, displaying a visual graphical userinterface pattern, determining an intermediate luminance point as basedupon user input in reaction to the visual graphical user interfacepattern, and displaying a ramp target of patches of known input valuesincluding the determined intermediate luminance point on the displaydevice. This is followed by capturing an image of the ramp target ofpatches with a digital camera, and calibrating the digital camera asbased upon the captured image of the ramp target patches and thedetermined intermediate luminance point. That is followed by extractingcalibrated camera signals from the captured image of the ramp target ofpatches, the extracted calibrated camera signals corresponding to thepatches in the ramp target of patches, and deriving a tone responsecalibration for the display device from the extracted calibrated camerasignals and the known input values.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a system representation suitable for performing theteachings provided herein.

FIG. 2 depicts a simple flow chart for color calibration.

FIG. 3 shows a graph of luminance response for a given input signal intoeither a CRT or alternatively a projection LCD display.

FIG. 4 depicts one possible color calibration interface for userinteraction.

FIG. 5 shows a normalized graph of luminance response for a given greeninput signal response for an on site camera versus the true cameraversus sRGB.

FIG. 6 shows an exemplary target embodiment suitable for performing theteachings provided herein

FIG. 7 shows a graph providing a comparison of calibration results.

DETAILED DESCRIPTION

A methodology is herein taught for calibrating an output deviceincluding a display, using a digital camera as a color measurementdevice. It is to be understood that the term “display” may include thecathode ray tube (CRT), desktop liquid crystal display (LCD), projectionLCD, digital light projector (DLP), and other similar technologies. Itwill also be understood by those skilled in the art that the term“digital camera” may include a standard digital still camera,camera-phone, video camera with still image capture, web camera, andother similar technologies. To explain the teachings provided herein,embodiments using a projection display and digital still camera will beused as example devices. As shown in FIG. 1, a target of known RGBvalues 100 is projected on the screen 110 by projector 120, and capturedwith the digital camera 130. The collected camera signals 140 are thencorrected in processor 150 to produce luminance signals, and the latterare used to calibrate the tone response of the projector 120. A numberof techniques exist to correct the collected camera signals 140. Onepreferred embodiment performs an “on-site” camera 130 based correctionfrom the said projected target 100. The advantages with this methodologyinclude: 1) sufficiently accurate tone response correction which isentirely satisfactory in most applications; and, 2) the use of a commonconsumer digital camera 130, thus eliminating the need for costly andtedious measurement tasks.

Display devices 120 typically conform to an additive color mixing model.According to this model, the relationship between RGB signals drivingthe device 120, and XYZ tri-stimulus values produced by the display isas shown in FIG. 2. The first step to be performed (in one preferredembodiment by processor 150) is a tone response calibration 200, whichlinearizes each of the R, G, and B channels to luminance. In a secondstep 210, the linearized signals, R′, G′, B′ are related to XYZtri-stimulus values 220 via a 3×3 characterization matrix, determined bythe colors of the R, G, B phosphors and the display white point. Forgreatest accuracy, both the tone calibration and the 3×3 matrix shouldbe derived for each display 120. However for many practicalapplications, entirely sufficient accuracy is achieved by deriving onlythe tone calibration, and using a fixed generic 3×3 characterizationmatrix such as the sRGB standard. Thus the teachings provided hereinfocus on a tone response calibration 200.

The tone response of a typical CRT is accurately modeled by agamma-offset-gain (GOG) model. A common simplification is to assumeoffset=0, gain=1. This reduces the model to:R′=R^(γ) G′=G^(γ) B′=B^(γ)  (1)where R, G, B and R′, G′, B′ are normalized to the range 0-1, and theexponent γ is often referred to as “gamma”. The curve 300 in FIG. 3 is aplot of Equation (1) with γ=2.2.

Due to the predominance of CRT displays in the past, it has been commonpractice to prepare electronic RGB images for rendition to such devices.In recognition of this fact, the sRGB color space was developed torepresent an average CRT display, and serves today as the main de-factostandard for electronic RGB imagery. Indeed many scanner and digitalcamera manufactures apply post-processing to the captured images totransform them approximately to sRGB. The CRT plot 300 in FIG. 3 is aclose approximation of the sRGB tone response.

Digital projection displays 120 are commonly used for giving electronicpresentations. Several technologies are available, of which liquidcrystal displays (LCD) are perhaps the most common. Although LCDsconform to the same basic additive model shown in FIG. 2, their toneresponse characteristics can be markedly different from that of CRTs.The projection LCD curve 310 in FIG. 3 is the tone response of a typicalportable LCD projector. The plot was derived from radiometricmeasurements of 11 neutral (R=G=B) patches projected on the screen underdark-room conditions. The difference between the tone response of theprojection LCD and CRT is quite apparent. The consequence is that if ansRGB image, prepared for display on a CRT, is rendered directly to aprojection LCD 120 (as is commonly done today), the reproduction isgrossly incorrect. This level of image quality is clearly unacceptablein cases where the color reproduction is critical to the value of thepresentation. Examples include technical, educational, and marketingpresentations attempting to demonstrate subtle color and image qualityeffects.

A method is therefore needed to accurately calibrate the projector'stone response. This requires the following basic steps:

-   1) Establish the built-in projector settings (typically default) and    viewing environment (typically a dim or dark-lit room)-   2) Generate a color target of known device values. The target should    comprise ramps in gray (R=G=B) and/or the primary R, G, B axes.-   3) Project the target onto the screen and take device-independent    color measurements of the patches.-   4) Relate the device values to the device-independent values via a    tone response calibration function.    Several techniques exist to accomplish the above steps, as are    discussed below.

A standard approach for determining the projector's tone response is tomake device-independent measurements of R, G, B ramps with aspectroradiometer, and then derive a tone response function that relatesdigital input value to luminance by fitting or interpolating themeasured data [see for example: Y. Kwak, L. W. MacDonald, “Method ForCharacterising An LCD Projection Display”, Projection Displays VII, SPIEProceedings 4294, pp. 110-118, 2001]. The authors J. Hardeberg, L.Seime, T. Skogstad, in their writing “Colorimetric Characterization OfProjection Displays Using A Digital Colorimetric Camera” augment thespectroradiometer with a calibrated digital camera to correct forspatial non-uniformities in the projected image. This approach isexpected to produce a highly accurate correction. However, makingspectroradiometric measurements is a very expensive, time-consuming andtedious process. Indeed this is the reason why projection displaycalibration is usually avoided, and users simply live with and otherwisetolerate the result.

An alternative to measurement-based approaches is visual calibration. Anexemplary example of a display for visual calibration 400 applied toCRTs is shown in FIG. 4. For each of the R, G, and B primaries, acorresponding red (410), green (420), and blue (430), GUI panel withslider 440 is provided. The left field 450 of each panel (410, 420, &430) contains a pattern of alternating lines of black in combinationwith the full-strength primary. Thus the average luminance of the left450 field is 50% between that of black and full-strength primary, and isthereby a known constant (it will be apparent to those skilled in theart that some other intermediate point other than 50% could be chosen).The user is asked to move the slider 440 to adjust the digital inputprovided to the right field 460 until the two fields (450 & 460) matchvisually in luminance. This task establishes one [x-y] pair on thedisplay tone response curve. If one assumes the simplified CRT model inEquation (1) above, this information is sufficient to determine thegamma parameter, which in turn defines the entire tone response.

The visual task in FIG. 4 may be successful for CRT calibration.However, as noted earlier, projection displays often exhibit an“S-shaped” tone response rather than a power-law response. Therefore, anattempt to fit a power-law model to a projector response using thetechnique in FIG. 4 will produce an incorrect tone calibration. Theaforementioned visual technique can be extended to estimate multiplepoints on the tone response curve. However, this necessarily involvesrepetitions of the visual tasks in FIG. 4, which can become tedious anderror-prone.

Thus an exemplary method is proposed for projection display calibrationthat addresses the problems that occur with these techniques. The same 4basic steps described above are still followed. However, a digitalcamera is used instead of a spectroradiometer to obtain the targetmeasurements in Step 3. This methodology is distinct from the prior artand technique (as for example in J. Hardeberg, L. Seime, T. Skogstad,“Colorimetric Characterization Of Projection Displays Using A DigitalColorimetric Camera”) in that here the digital camera is the onlymeasurement device used for calibrating the projector 120. Furthermore,the exemplary method does not require a sophisticated camera—a commonconsumer device will suffice.

Advantages with this approach are:

-   i) consumer digital cameras abound today as an inexpensive commodity    item;-   ii) digital cameras are easy to use in comparison to spectral    measurement devices;-   iii) a digital camera can capture a fairly large spatial footprint,    thus allowing for measurement of a large number of patches, and/or    greater spatial averaging.    However, Issues to address with this approach are:-   a) since device-independent measurements are needed, the camera    itself needs to be calibrated for the projected medium it is    capturing;-   b) the camera may drift over time, thus invalidating the data it    captures.

Digital camera calibration and characterization has engendered a largebody of research literature. As mentioned earlier, manufacturers ofconsumer cameras often incorporate a built-in correction to produceimages in a standard space (often sRGB). As a test to illustrate this,FIG. 5 compares the sRGB tone response (curve 500) with the toneresponse of the green channel from a Kodak LS443 digital camera (curve510). The latter was obtained by displaying a gray ramp with the abovementioned LCD projector, and capturing both a digital camera image, andluminance measurements with a PhotoResearch SpectraScan PR705spectroradiometer. Clearly, the camera tone response deviates noticeablyfrom the sRGB assumption. Furthermore, the response is likely to varywith the particular camera model, camera settings, image captureconditions, and over time. While these factors may not be an issue forcasual consumer needs, they may pose a problem in the application athand, where the digital camera is used as a measurement device.Obtaining calibrated signals from a digital camera requires that thecamera itself be calibrated. Many standard approaches exist for cameracalibration. However these techniques require spectral or colorimetricmeasurements of a suitable target, thus making the camera-based approachno less expensive and skill intensive than the originalmeasurement-based display calibration approach. It is thereforepreferable to perform some form of “on-site” camera calibration using aprojected target, and requiring no spectral or colorimetric measurement.

To this end, an exemplary technique is employed that uses the visualcalibration technique described above to calibrate the camera toneresponse. This is based on the assumption that the camera response canbe approximated by a representation akin to the GOG model for CRTs.Recall that the visual task in FIG. 4 produces one calibrated point;i.e. we know the input digital value to the projector that produces the50% luminance measurement. This point can be included in the projectedtarget and used to calibrate the camera.

To illustrate this, consider an exemplary target 100 shown in FIG. 6.This embodiment of the target comprises a ramp of 15 neutral (R=G=B)patches from white to black. One of the patches is the known 50%luminance point obtained from a visual calibration 400 (far right patch460 in all rows). This patch is duplicated along the entire middle row,and the far-right column, to optionally correct for spatialnon-uniformity in the displayed image in the horizontal and verticaldirections, respectively. This target 100 is displayed with theprojector 120, captured with the digital camera 130, and the camera RGBvalues 140 are retrieved.

Correction for spatial non-uniformity is an optional step that can becritical for certain displays and cameras. The correction should ideallybe a spatial function applied to the captured camera image. However,this approach cannot be implemented with standard color managementarchitectures such as established by the International Color Consortium.A simpler alternative is to pre-correct the camera signals toapproximate the effect of displaying each patch at a single chosenreference location. This allows calibration to be derived fromwell-behaved data, although it is strictly valid only at the referencelocation. One exemplary method of spatial nonuniformity correctionapplied to a camera signal C(i,j) corresponding to the patch located atrow i and column j in the target is given by:C′(i,j)=C(i,j)*S ₁(j)*S ₂(i)  (2)where C′(i,j) is the camera signal corrected for spatial nonuniformity,and S₁ and S₂ are spatial correction factors in the horizontal andvertical directions, respectively. S₁ is derived from camera signalsobtained from the constant-input row of patches in the target.Similarly, S₂ is derived from camera signals obtained from theconstant-input column of patches in the target. One example of ahorizontal correction factor is given by:S ₁(j)=C(i _(const) , j _(ref))/C(i _(const) ,j)  (3)where i_(const) refers to the index of the constant-input row, andj_(ref) is the column index for the reference location. In the exampletarget of FIG. 6, the constant-input row is the second row, thereforei_(const)=2. If the reference patch is chosen to be near the center ofthe target, for example the patch in the second row and fourth column,then j_(ref)=4. An analogous formulation applies for S₂ in the verticaldirection. The aforementioned spatial non-uniformity correction can beapplied to the camera signals before or after camera calibration isapplied. This operation is described next.

Three points from the captured target are used to calibrate the camera130: namely white, black, and the 50% luminance point. In addition,perfect black (i.e. 0 luminance) is used to pin the one endpoint of thecamera 130 response. Table 1 summarizes the data used to calibrate thecamera 130 response. Luminance is normalized to that of projector white,so that by definition, Y_(w)=1. The only unknown parameter is theluminance of the projector black point, Y_(b). This flare factor isaffected by the characteristics of the projector 120, screen 110, andthe ambient room illumination. We assume 2% flare (i.e. Y_(b)=0.02)based on empirical a priori radiometric measurements from differentprojectors in a dim surround. (This parameter can be tuned based onadditional knowledge of the projector and viewing environment.)

TABLE 1 Data used to calibrate the tone response of the digital cameraPatch Luminance Captured camera signal Projector white Y_(w) = 1 R₁, G₁,B₁ Projector black Y_(b) R₂, G₂, B₂ Mid-gray (Y_(w) + Y_(b))/2 = R₃, G₃,B₃ (1 + Y_(b))/2 Perfect black 0 0, 0, 0

The four points in Table 1 can then be used to determine therelationship between camera RGB 140 and luminance. One approach is tofit a GOG model to the data. An empirical alternative is to simplyinterpolate the four points. Due to its simplicity, we adopted thelatter approach with cubic spline interpolation. The dashed line 520 inFIG. 5 shows the camera 130 tone response derived from this approach.Comparing this to the true camera response (black curve), we note thatthe technique is very accurate.

Once the camera 130 is calibrated, it is effectively turned into aluminance measurement device. Thus the luminance of all 15 patches inthe projected exemplary gray ramp target 100 depicted in FIG. 5 can bederived. These luminance values and the corresponding digital valuesdriving the projector 120 are then used to generate a tone responsecalibration 160 for the projector using straightforward interpolationtechniques. In our implementation a cubic spline was used to interpolateamong the 15 points from the target.

The benefit of this approach is that since the same target 100 is usedto calibrate both the camera 130 and the projector 120, the dependenceof the camera 130 response on capture conditions (i.e. projection media,image content, camera settings, etc.) is effectively calibrated out. Theexemplary correction technique is thus very robust to projection andcapture conditions.

FIG. 7 is a plot of the luminance response along the neutral (R=G=B)axis for one sample projector 120 as calibrated with differenttechniques. For reference, an identity transform is included as the thinblack line 730, and represents perfect calibration. The visualcalibration technique 700 is clearly inadequate as explained above.Camera-based calibration 710, wherein the camera is simply assumed to bean sRGB device, is also inadequate as it does not adequately linearizethe projector. (The invalidity of the sRGB assumption will vary acrossdifferent cameras) Finally, we note that the on-site camera calibration720 achieves exemplary performance in terms of linearizing theprojector.

To recapitulate, an exemplary methodology as taught herein provides anintegrated calibration tool that accomplishes the following process:

-   1. The calibration tool displays a visual GUI pattern 400 (e.g.    FIG. 4) on the screen 110.-   2. The user performs the required visual tasks to establish the 50%    luminance point.-   3. The calibration tool displays a ramp target 100 (e.g. FIG. 6) on    the screen. The 50% point from step 2 is included in this target    100.-   4. The user captures an image of this target 100 as projected onto    screen 110 with a digital camera 130, and downloads that captured    image data 140 to the software calibration tool resident on    processor 150.-   5. The software calibration tool resident on processor 150: a)    extracts RGB patch values from the camera image 140; b) calibrates    the camera response with selected patch values; c) converts the    camera responses to luminance values using the camera calibration;    and d) uses the digital values driving the projector and the    luminance values from step c) for all patches to create a tone    response correction (TRC) 160 for the projector 120.-   6. The TRC 160 is exported for subsequent correction of images and    documents to projector 120.    This calibration tool can be built with a Java GUI (Graphical User    Interface) and underlying software functionality. The software    calibration tool may either reside on the host computer driving the    projector 150 or on a remote server. Step 6 as presented above can    be accomplished by building an ICC profile for the projection    display 120, which can then be invoked by the operating system or    other applications such as page description language software, e.g.    Adobe® Acrobat® to create a PDF file. Alternatively, if the video    LUTs (Look Up Tables) driving the projection display 120 are    accessible, the projector 120 may be directly corrected. In this    case, the projector 120 could be turned into an “sRGB emulator”,    thus properly reproducing most existing color imagery.

As will be clear to one skilled in the art, the calibration tool may beprovided as a software platform, a software platform operating on ahardware platform or even provided as hardwired logic. The calibrationtool may be resident on an outboard personal computer or providedinboard of the projector. In the latter case the digital camera wouldnecessarily connect directly to the projector for the above calibrationmethodology to be performed.

The claims, as originally presented and as they may be amended,encompass variations, alternatives, modifications, improvements,equivalents, and substantial equivalents of the embodiments andteachings disclosed herein, including those that are presentlyunforeseen or unappreciated, and that, for example, may arise fromapplicants/patentees and others.

1. A method of correcting colors input to an output device, comprising: adjusting, based on a user's input, an output device to correctly display, at a predefined known luminance level, an input target having a predefined calibration input value corresponding to the predefined known luminance level; rendering, on the output device subsequently to the adjusting, a target of patches, each patch in the target of patches having a respective known input value, the target of patches comprising: at least one base patch having the predefined calibration input value, thereby resulting in the at least one base patch being displayed by the rendering on the output device with the predefined known luminance level; and a plurality of calibration patches comprising separate patches each having a respective input value corresponding to a different known respective luminance value, wherein patches within the plurality of calibration patches are displayed by the rendering on the output device with respective uncalibrated luminance levels that are different from the predefined known luminance level; capturing, with a common digital camera, a captured an image of a rendering of the target of patches produced by the rendering; extracting camera signals from the captured image for each of the plurality of calibration patches and the at least one base patch; deriving a tone response calibration for the output device based upon a measured relationship between the camera signals extracted for each of the plurality of calibration patches and their respective input values relative to the camera signals extracted for the at least one base patch and the predefined calibration input value; and applying the derived tone response calibration to the output device so as to provide corrected colors therefrom.
 2. The method of claim 1 wherein the at least one base patch is duplicated at multiple spatial locations in the target of patches, and the camera signals extracted for the at least one base patch are used to correct for spatial non-uniformity.
 3. The method of claim 1, wherein the adjusting comprises: displaying, on an output device, an luminance calibration pattern comprising a known luminance field and an adjustable luminance field, the known luminance field having a predefined average luminance percentage point for the output device that is equal to the predefined known luminance level, and the adjustable luminance pattern having an adjustable luminance percentage point controlled by an adjustable input value; and accepting an input from the user, in response to the displaying, to adjust the adjustable input value to have a matching input value, the matching input value resulting in the adjustable luminance percentage point matching the predefined average luminance point.
 4. The method of claim 1, wherein the plurality of calibration patches comprises a zero luminance patch with a respective input value corresponding to zero output luminance and a full luminance patch with an input corresponding to full output luminance, and wherein the deriving a tone response calibration for the output device comprises: extracting portions of the captured image corresponding to the zero luminance patch and the full luminance patch to determine respective displayed luminance levels; and defining the tone response calibration by fitting a response curve through a displayed luminance level of the zero luminance patch, the predefined known luminance level, and a displayed luminance level of the full luminance patch.
 5. The method of claim 1, wherein the deriving comprises: generating a camera calibration function based on a response of the common digital camera to the at least one base patch in the captured image; and calibrating, prior to the deriving, the camera signals extracted for the for each of the plurality of calibration patches and the at least one base patch by processing the camera signals through the camera calibration function.
 6. A system for color correction for a display device, comprising: a point display adjuster adapted to adjust, based on a user's input, a display device to correctly display, at a predefined known luminance level, an input target having a predefined calibration input value corresponding to the predefined known luminance level; a pattern generator adapted to display, on the display device subsequently to the adjusting, a target of patches, each patch in the target of patches having a respective known input value, the target of patches comprising: at least one base patch having the predefined calibration input value, thereby resulting in the at least one base patch being displayed by the rendering on the output device with the predefined known luminance level; and a plurality of calibration patches comprising separate patches each having a respective input value corresponding to a different known respective luminance value, wherein patches within the plurality of calibration patches are displayed by the rendering on the output device with respective uncalibrated luminance levels that are different from the predefined known luminance level; a common digital camera adapted to capture a captured image of a rendering of the target of patches produced by the rendering; an image processor adapted to extract camera signals from the captured image for each of the plurality of calibration patches and the at least one base patch; a processor adapted to derive a tone response calibration for the display device based upon a measured relationship between the camera signals extracted for each of the plurality of calibration patches and their respective input values relative to the camera signals extracted for the at least one base patch and the predefined calibration input value, and wherein the user display is further adapted to apply the derived tone response calibration so as to provide corrected colors therefrom.
 7. The system of claim 6 wherein the display device is a projection display.
 8. The system of claim 6 wherein the display device is a LCD type display.
 9. The system of claim 6 wherein the at least one base patch in the target of patches is duplicated at multiple spatial locations in the target of patches, and the camera signals extracted for the at least one base patch are used to correct for spatial non-uniformity.
 10. The system of claim 6 wherein the camera signals extracted by the image processor include a luminance signal.
 11. A method of color correction for a display device, comprising: displaying a visual graphical user interface pattern on a display device; adjusting, based on a user's input in response to the displaying the visual graphical user interface pattern, the display device to correctly display, at a predefined known intermediate luminance level, an input target having a predefined calibration input value corresponding to the predefined known intermediate luminance level; displaying, on the display device subsequently to the adjusting, a ramp target of patches, each patch in the ramp target of patches having a respective known input value, the ramp target of patches comprising: at least one base patch having the predefined calibration input value, thereby resulting in the at least one base patch being displayed by the rendering on the output device with the predefined known luminance level; and a plurality of calibration patches comprising separate patches each having a respective input value corresponding to a different known respective luminance value, wherein patches within the plurality of calibration patches are displayed by the rendering on the output device with respective uncalibrated luminance levels that are different from the predefined known luminance level; capturing, with a common digital camera, a captured image of the ramp target of patches; extracting camera signals from the captured image for each of the plurality of calibration patches and the at least one base patch within ramp target of patches, deriving a tone response calibration for the display device based upon a measured relationship between the camera signals extracted for each of the plurality of calibration patches and their respective input values relative to the camera signals extracted for the at least one base patch and the predefined calibration input value; and applying the derived tone response calibration to the output device so as to provide corrected colors therefrom.
 12. The method of claim 11 wherein the predefined known intermediate luminance level is halfway between the luminances produced by the minimum and maximum input values to the display device.
 13. The method of claim 11 wherein the display device is a projection display.
 14. The method of claim 11 wherein the display device is a LCD type display.
 15. The method of claim 11 wherein the at least one base patch in the ramp target is duplicated at multiple spatial locations on the ramp target, and the camera signals extracted for the at least one base patch are further used to correct for spatial non-uniformity within the captured image of the ramp target.
 16. The method of claim 11 wherein deriving a tone response calibration for the display device includes fitting, with a spline function, the relationship between the respective input value of each patch to the display device and the camera signals.
 17. The method of claim 11 wherein the step of extracting camera signals includes computing luminance from RGB values extracted from the captured image of the ramp target of patches.
 18. The method of claim 11 wherein the tone response correction is exported to the display device.
 19. The method of claim 11 wherein the tone response correction is exported to a host computer driving the display device.
 20. The method of claim 19 wherein the tone response correction exported to the host computer is invoked by an application.
 21. The method of claim 20 wherein the application is page description language software. 